Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibers in their networks as efficiently as possible, in particular for communication over long distances, since such fibers obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibers and in new communication systems to be built a plurality of individual wavelength channels can be transmitted on the same optical fiber and thus the information transmitted over the fiber can be multiplied.
In optical fiber networks for long distance communication there may be a need for amplifying the optical signals. Such amplification can of course be achieved by a repeater built in a straight-forward way, including components converting the optical signals to electrical signals, amplifying the electrical signals and converting the electrical signals to optical signals. For WDM signals this will require one optoelectrical and one electrooptical converter per wavelength channel used in the WDM transmission and also one filter or demultiplexer for filtering out the different wavelengths in the incoming signal. This will obviously be very costly and also results in reliability problems owing the large number of components, both electronic and optical, which are required.
Another type of amplifier comprises optical fiber amplifiers based on optical fibers doped with rare-earth metals, primarily erbium-doped fiber amplifiers. Such amplifiers have great advantages when used in optical fiber systems owing to e.g. their compatibility with the optical fibers and their high gain, and they are in particular advantageous when used in wavelength multiplexed transmission systems, since they are capable of simultaneously amplifying a number of WDM channels and only require a limited amount of electronic components. The basic design of an erbium-doped fiber amplifier includes one length of an active, erbium-doped optical fiber, connected at its input end to the output of a 2-to-1 optical coupler, the coupler receiving on one of its inputs the signal to be amplified and on the other input more energetic light providing the power for amplifying the signal. This more energetic input light is called the pump light and is obtained from an optical power source, called the optical pump. The pump light has a shorter wavelength than that of the signal and is generally more energetic and is capable of lifting erbium ions from lower energy states to higher energy states in the erbium-doped fiber. Light is then generated in the fiber when the ions return to lower energy levels.
In a typical WDM system the number of used wavelength channels normally varies at random. The gain of an erbium-doped fiber amplifier operated in the conventional saturated way is dependent on the number of wavelength channels, see the diagram shown in FIG 1, owing to the total constant output power of the amplifier. In FIG. 1 the gain for a typical erbium-doped fiber amplifier is plotted as a function of wavelength of the case of only using one WDM wavelength channel, see the upper curve, and for the case of all channels being active or used, see the lower curve. The gain has a difference of at least about 10 dB for the important wavelength band of 1540-1560 nm. This means that also the gain will vary at random when the erbium-doped fiber amplifier is used in a typical WDM system used for telecommunication.
In WDM systems it is obviously important to have a constant and flat output gain in the used wavelength band, since this will allow e.g. an optimization of other components. Also, it is inefficient to use gain flattening filters, see e.g. the article by Paul F. Wysocki et al., "Broad-band Erbium-Doped Fiber Amplifier Flattened Beyond 40 nm Using Long-Period Grating Filter", IEEE Photonics Techn. Lett., Vol. 9, 10, Oct. 1997, since such filters must be designed according to the actual gain curve of the amplifier. A possible solution would be to control the pump power provided to the active fiber, but this includes disadvantages resulting from the long lifetime of excited erbium ions in the active fiber.
In the published European patent application 0 777 346 an optical amplifier used in a communication system is disclosed comprising an optical amplifying medium, i.e. a length of an erbium doped optical fiber, a pump light source and a probe light source, which sources both inject light into the amplifying medium. The probe light has a wavelength included in the amplification band of the optical amplifying medium and is preferably different from the wavelength of the signal light. Control means maintain the gain for the input signal constant. In the control the power of the probe light and the power of the input signal light are added to each other, giving the powers suitable weights in the adding operation. The result of the weighted addition is controlled to be constant by adjusting the power of the probe light.